1. Field of the Invention
This invention relates to a cap for protecting and sealing electronic components, more particularly, a discrete cover for sealing one or more integrated circuit chips or dies mounted on a printed circuit board.
The protection and sealing of electronic components such as an IC die is necessary due to the fineness and low structural strength of bonding wires leading from the die to contact pads on the metallization patterns of the printed circuit board and the need to prevent moisture and other contaminants from reaching the bonding wire joint and the interior of the die package.
2. Description of the Prior Art
From early on, electronic components have been potted by various insulating sealants within cans or provided with insulating envelopes surrounding the component(s). These methods, however, create mechanical stresses on the components, particularly on any wiring leading from the component. With the advent of microminiaturized integrated circuit devices and other micro-components, the problem of protecting the small-sized electrical connections necessitated by the microsized metallization patterns and contacts has been increasing. At the same time there is a necessity to increase yields in the fabrication process and a need for increased quality in the final product. In the case of integrated circuit dies the present state of the art involves utilizing a rectangular, normally square, flat-topped cap or box which is positioned over and around the die after the die has been affixed and bonding wires connected to the printed circuit board. Such cap or box may also have staking pins extending from its bottom edges at the open bottom of the cap, the pins being inserted into matching apertures in the circuit board for initially holding the cap in place. A liquid epoxy sealant is then flowed from a manually-operated syringe or complicated automated dispensing system, around the bottom periphery of the cap so that a sealing miniscus is formed between the exterior of the cap sides and the circuit board top surface.
In the above technique, it is difficult to control the flow of the dispense sealant. It has been found that use of the above-described caps often result in component failures due to inconsistent, uneven flow and sealing of the sealant and the susceptibility of the seal to break when the circuit board is subject to warpage or bending. Further, it has been found that the rectangular caps themselves may be subject to sufficient downward heat deformation, particularly at their center, bending or breaking off the delicate bonding wires leading from the IC die underneath the bottom surface of the top of the cap. The mounting and sealing of the present caps are difficult to automate, requiring special tooling and additional equipment. Further, without extreme care by the operator, the operation can become very messy with the sealant flowing over unwanted areas of the circuit board, into plate-through holes and otherwise causing unneeded problems. A relatively high material wastage has been experienced due to the use of liquid viscous sealing material. Even in satisfactory use they require a relatively large volume of sealant with resultant high costs since the miniscus or bead has a relatively large cross-section which extends over a relatively large area on the cap sides and then flows outwardly over a relatively large lateral area of the printed circuit board. This also affects the closeness that contact pins and plated through-holes can be placed with respect to the die dictating a larger size board for the desired components/patterns to be placed on the board.